validating the date format, add test case, since NumberFormatExceptio…

gson/src/main/java/com/google/gson/GsonBuilder.java

@@ -43,6 +43,7 @@
 import com.google.gson.stream.JsonWriter;
 import java.lang.reflect.Type;
 import java.text.DateFormat;
+import java.text.SimpleDateFormat;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -598,7 +599,14 @@ public GsonBuilder disableHtmlEscaping() {
    */
   @CanIgnoreReturnValue
   public GsonBuilder setDateFormat(String pattern) {
-    // TODO(Joel): Make this fail fast if it is an invalid date format
+    if (pattern != null) {
+      try {
+        new SimpleDateFormat(pattern);
+      } catch (IllegalArgumentException e) {
+        // Throw exception if it is an invalid date format
+        throw new IllegalArgumentException("The date pattern '" + pattern + "' is not valid", e);
+      }
+    }
     this.datePattern = pattern;
     return this;
   }

gson/src/main/java/com/google/gson/internal/bind/util/ISO8601Utils.java

@@ -289,11 +289,7 @@ public static Date parse(String date, ParsePosition pos) throws ParseException {
       return calendar.getTime();
       // If we get a ParseException it'll already have the right message/offset.
       // Other exception types can convert here.
-    } catch (IndexOutOfBoundsException e) {
-      fail = e;
-    } catch (NumberFormatException e) {
-      fail = e;
-    } catch (IllegalArgumentException e) {
+    } catch (IndexOutOfBoundsException | IllegalArgumentException e) {
       fail = e;
     }
     String input = (date == null) ? null : ('"' + date + '"');

gson/src/test/java/com/google/gson/functional/DefaultTypeAdaptersTest.java

@@ -16,6 +16,7 @@
 package com.google.gson.functional;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.fail;
 
 import com.google.gson.Gson;
@@ -737,6 +738,24 @@ public void testStringBufferDeserialization() {
     assertThat(sb.toString()).isEqualTo("abc");
   }
 
+  @Test
+  public void testSetDateFormatWithInvalidPattern() {
+    GsonBuilder builder = new GsonBuilder();
+    String invalidPattern = "This is a invalid Pattern";
+    assertThrows(
+        IllegalArgumentException.class,
+        () -> {
+          builder.setDateFormat(invalidPattern);
+        });
+  }
+
+  @Test
+  public void testSetDateFormatWithValidPattern() {
+    GsonBuilder builder = new GsonBuilder();
+    String validPattern = "yyyy-MM-dd";
+    builder.setDateFormat(validPattern);
+  }
+
   private static class MyClassTypeAdapter extends TypeAdapter<Class<?>> {
     @Override
     public void write(JsonWriter out, Class<?> value) throws IOException {